it is well known that many dangerous communicable diseases are spread through contacting the body fluids of an infected person. After use of a syringe, residual body fluids are likely to remain on or within the syringe needle. For this reason, syringes are typically intended for a single use only. In order to be handled safely after use, the needle of a syringe must be covered to prevent it from accidentally stabbing a person who is, for example, collecting the syringe for disposal, thereby releasing residual body fluids into such person. Typically, a protective cap is provided with the syringe, which after use of the syringe, can be used to cover the tip of the needle. However, it sometimes happens that persons attempting to cap a used needle miss the cap and accidentally stab themselves, resulting in potential exposure to communicable diseases. Further, spread of communicable and dangerous diseases is effected by drug-addicted individuals sharing and re-using needles and syringes intended for single use.
There have been several attempts to address this problem by incorporating into syringes, mechanisms for retracting the needle into the syringe following use. U.S. Pat. No. 5,334,155 (Sobel, 2 Aug. 1994) discloses a needle guard comprising an evacuated double walled protective sheath. Before use, the partial vacuum within the protective sheath causes the sheath to fold inwardly upon itself so that the needle extends beyond the protective sheath and may be used for injections. Subsequent to injection, the double wall of the protective sheath can be breached in one place so that the inside of the protective sheath reaches atmospheric pressure. The protective sheath then extends to cover the projecting needle. However, the protective sheath may interfere with use of the syringe as it may obstruct the view of the point the needle is to be inserted into the patient. In addition, it is inconvenient to use; after injection, the user must change the user's hand position on the syringe in order to breach the double wall and activate the sheath. In this manner activation of the safety mechanism is not automatic following injection of the medicament.
The protective safety device shown in U.S. Pat. No. 5,188,614 (Hart, 23 Feb. 1993) is a hollow cylindrical casing that encompasses the syringe. A dual component foaming agent is disposed at the downstream end of the casing. Following injection, the two components of the dual component foaming agent are mixed, creating an expanding foam mixture that forces the syringe back within the casing and encompasses the needle. However, this device suffers from the disadvantages that the casing may interfere with the use of the syringe in making injections as it is designed to fit over a conventional syringe thereby changing the size and feel of the device as compared to a conventional syringe. In addition, a considerable amount of material is necessary in order to make the protective sheath, increasing the expense of both making and disposing of the device.
U.S. Pat. No. 6,193,695 (Rippstein, 27 Feb. 2001) discloses a safety syringe comprising a vacuum chamber on the upstream side of the plunger head. Following injection of medicament, the plunger head engages the needle head, the ambient atmospheric pressure external to the needle head acts on the needle head, forcing the needle and plunger back against the vacuum into the syringe body. The plunger arm may then be snapped off by the user to prohibit further use of the needle. This device suffers from the disadvantage that accidental re-extension of the needle is possible if the plunger arm is not snapped off by the user. A further disadvantage of this device is that if the user does not apply a constant injection force, there is the possibility that the plunger will retract under the vacuum before the medicament is completely injected, thereby causing the syringe to work in reverse.
U.S. Pat. No. 6,413,236 (Van Dyke, 2 Jul. 2002) discloses a safety syringe comprising a vacuum chamber on the upstream side of the plunger head. Following injection of medicament, the plunger head engages the needle head, and the ambient atmospheric pressure external to the needle head acts on the needle head, forcing the needle and plunger back against the vacuum into the syringe body. In this patent, in contrast to U.S. Pat. No. 6,193,695, the needle is lodged in the syringe body at an angle so that the piercing tip end of the needle is pressed against the inner surface of the syringe prohibiting re-extension of the needle even though the plunger arm is fully extended outside the syringe body. However, this device still has the disadvantage that if the user does not apply a constant injection force, there is the possibility that the plunger will retract under the vacuum before the medicament is completely injected, thereby causing the syringe to work in reverse.
The device disclosed in U.S. Pat. No. 5,868,713 (Klippenstein, 9 Feb. 1999) embodies a significant improvement over the previous syringe technology. This earlier Klippenstein syringe includes a gas reservoir that contains a non-toxic compressed gas. Once the gas reservoir is ruptured by the needle header when the needle header is forced in a downstream direction, the released non-toxic compressed gas provides an upstream biasing pressure that biases the needle header and plunger to slide upstream, retracting the needle into the syringe body. A locking mechanism prevents downstream motion of the plunger after retraction of the needle. However, at the end of the retraction phase, the plunger continues to extend outside the barrel of the syringe, thus requiring the user to have changed his hand position by moving his thumb away from the thumb button at the end of the plunger. Further, due to the retention of the plunger in the locked extended position after retraction, the extended syringe takes more space in disposal containers than do conventional syringes. It is an objective of the present invention to overcome these and other disadvantages associated with this prior Klippenstein design.
A syringe designed to provide an optimal solution to the problem of prevention of accidental needle stabbing after the use of the syringe for injection would include the following characteristics:    1. The syringe mechanism should be relatively simple, in that it should be made from as few moving parts as possible consistent with its design objectives, and should be simple to operate, preferably with the look and feel of a conventional syringe.    2. The syringe mechanism should reliably retract the needle or otherwise reliably shield the needle after use, so that accidental stabbing is prevented.    3. The syringe should be relatively inexpensive to manufacture.    4. There should be a minimum of waste plastics and other materials to be disposed of after use of the syringe.    5. Safety-related means should not appreciably interfere with the feel of the syringe in the user's hand.    6. Once the needle has been retracted or shielded, a reliable safety device should prevent the needle from becoming once again exposed.Note that simplicity of structure and operation, objective 1 above, may contribute to the achievement of the other five objectives listed above.